(1) Field of the Invention
This invention relates to the fabrication of integrated circuit devices and more particularly to methods of metallizing substrates which are then useful as microwave circuits.
(2) Description of the Prior Art
Thick film processing for microwave circuits is desirable because of the low equipment and production costs, lower electrical line resistance, and good adhesion between the conductor and the substrate.
The disadvantages of this method are a larger conductor structure size (smallest clearance and conductor width is 100 to 125 microns with an edge accuracy of 20 to 30 microns due to screen mesh size), larger surface roughness, poor thickness uniformity, higher electrical transmission loss, and lower circuit density.
The advantages of thin film processing are a smaller conductor size (clearance and conductor width is less than 10 microns with an edge accuracy of less than 1 micron), good surface roughness, lower electrical transmission loss, good thickness uniformity, and higher circuit density.
Thin film processing requires a higher investment in equipment, higher production costs, lower production rates, higher electrical line resistance, and poor adhesion between conductor and substrate.
A. Rose in "Thin Film Circuit Manufacturing using Thick Film Technology" in Hybrid Circuit Technology March, 1990 pp. 48-49 points out that the metallorganic material allows the fabrication of thin film circuits by print-and-fire technology for they leave an extremely smooth thin film on the substrate after printed and fired. Unfortunately, they can not be printed (or need to repeat many times) to an acceptable thickness. For example, if there is needed a layer of 10 micrometers in thickness and a resulting thin film after single application and firing only typically leaves a thickness of 0.3 to 0.6 micrometers, there would have to be 20 or more applications and firings to obtain a total layer thickness of 10 micrometers.
Another solution is plating the screen printed metallorganic layer. However, there are many disadvantages to this course which include high cost, complex process, more contamination problems and the plating solution must be either acid or alkaline, so there will be damage to the screen printed metallorganic material and degradation to its adhesion to the substrate.
Some of the prior technology is shown in U.S. Pat. No. 4,710,395 to W. M. Young et al wherein a vacuum table is used to pull the conductor material into the through holes. U.S. Pat. No. 4,416,932 to K. M. Nair describes a noble metal thick film composition with glass for sintering to form lines on a ceramic substrate. U.S. Pat. No. 4,748,742 to E. J. Choinski shows a vacuum table to pull material into and through the holes in the circuit board using significantly different materials than the present invention.